"No Voice" policy
No voice policy is a standard policy in ASL instruction. ASL is taught entirely in classes with a firm "no voice" policy. From the time you arrive in class, all conversation should be in ASL until you leave at the end of class.
The classroom is a speech-free zone for two major reasons: learning environment and cultural respect. Not only it is a classroom policy, it is also a cultural norm and custom in visual ("sighting") world. Learning a language is not without learning its culture, inseparable.
A student's learning ability is greatly enhanced by this no voice environment. Full immersion helps develop better receptive and expressive skills. Using voice may distract or interfere other students' learning process.
In the early stage of learning ASL (e.g. level 100), "no voice" policy is crucial. Some students may think it is easier to or may be tempted to learn ASL by using vocal English. Easy start but bad start -- it doesn't help in the long run in language skill. It's probably challenging for some of them in the beginning but a breakthrough will be rewarding in the long run. Those who choose the easier way in the beginning may face a more challenging receptive/expressive skills in the later stage.
Another reason is to maintain that ASL is a language of its own. Students are taught to think in ASL, not English while signing ASL. Using voice (English) while signing ASL may interfere with language development in vocabulary and grammar: incorrect uses of ASL vocabulary in different contexts (semantics), an incorrect grammatical structure and/or a limit of grammar skill.
Respect for a culture of the language
Classroom is a good place to train and learn to practice "no voice" habit that will be applied to a cultural behavior and norm in the ASL/Deaf world outside a classroom. Hearing people who know ASL usually talk in ASL in any ASL/Deaf space or in front of any Deaf person to respect their culture and language. The use of voice is a rude or offensive behaviour in Deaf space. It demonstrates an ignorance or disrespect for their culture as well as a reminder of the historical oppression.
Language and culture are inseparable, intergrated, and intertwined. ASL students are not just learning this language, you also learn their culture. Your role is to be an ally. For ASL is the most valued identity of their culture, the ASL natives whose language is regarded as the most precious gift of their culture share their language with you.
Learning ASL in class is usually a fun and interesting experience. Students often have a positive learning experience, but also make sure the ASL instructors have a positive teaching experience also. Respect is a peaceful language across all languages and cultures.